In production of a fluoropolymer such as polytetrafluoroethylene (hereinafter referred to as PTFE), a melt-processable fluororesin or a fluoroelastomer by emulsion polymerization, an anionic fluorinated emulsifier which will not inhibit the polymerization reaction by chain transfer in an aqueous medium is commonly employed.
By agglomerating and drying an aqueous emulsion of a fluoropolymer obtainable by emulsion polymerization (hereinafter referred to as a fluoropolymer aqueous emulsion), a powder of a fluoropolymer is obtained. The powder of a fluoropolymer is used for various applications after forming e.g. by paste extrusion. Further, a nonionic surfactant or the like is added as the case requires to the fluoropolymer aqueous emulsion for stabilization treatment, followed by concentration, whereby a fluoropolymer aqueous dispersion containing a fluoropolymer at a high concentration can be obtained. The fluoropolymer aqueous dispersion can be used for various coating applications, for impregnation application, etc., after various compounding agents or the like are added as the case requires.
By the way, the anionic fluorinated emulsifier to be used for emulsion polymerization of a fluoropolymer is a substance which is not easily decomposed in the natural world. Accordingly, in recent years, it is desired to reduce the anionic fluorinated emulsifier contained not only in industrial waste but also in products such as a fluoropolymer aqueous emulsion or a fluoropolymer aqueous dispersion.
As a method for reducing the anionic fluorinated emulsifier, a method of bringing a liquid to be treated containing the anionic fluorinated emulsifier into contact with a basic ion exchange resin, so that the anionic fluorinated emulsifier in the liquid to be treated is adsorbed on the basic ion exchange resin may be mentioned. Further, as the anionic fluorinated emulsifier is expensive, an attempt to recover the anionic fluorinated emulsifier adsorbed on the basic ion exchange resin and recycle it has been made.
For example, Patent Document 1 discloses that a basic ion exchange resin on which an anionic fluorinated emulsifier is adsorbed is treated with a mixture of a diluted mineral acid with an organic solvent to recover it as an acid of the anionic fluorinated emulsifier. It is disclosed that the organic solvent is preferably a solvent such that when it is mixed with the same amount of water, at least 40% thereof is miscible, or it is infinitely miscible.
Further, Patent Document 2 discloses that a basic ion exchange resin on which an anionic fluorinated emulsifier is adsorbed, is brought into contact with a mixture of water, a solvent such as methanol and/or dimethyl monoglycol ether or dimethyl diglycol ether, and an alkali metal hydroxide ammonia solution, to elute the anionic fluorinated emulsifier bonded to the basic ion exchange resin.
Further, Patent Document 3 discloses that a basic ion exchange resin on which an anionic fluorinated emulsifier is adsorbed, is treated with an aqueous alkali solution containing water and an organic solvent. It discloses that the organic solvent is one in which water is dissolved or which is dissolved in water, and is preferably one in which at least 10 vol % of water is dissolved.
Further, Patent Document 4 discloses that a basic ion exchange resin on which an anionic fluorinated emulsifier is adsorbed is treated with a water-miscible organic solvent containing at least one ammonia and having a boiling point less than 150° C. Specifically, a mixture of ammonia with methanol is used.